Theft of electronic devices, such as cell phones, tablet computers, etc., is a significant problem. Device manufacturers and cellular network providers have attempted several ways to thwart theft, including the use of an International Mobile Equipment Identity (IMEI) in 3GPP (3d Generation Partnership Project) phones, iDEN (Integrated Digital Enhanced Network) phones, and some satellite phones. The IMEI number is used by a GSM (Global System for Mobile Communications) network to identify valid devices and it can be used for stopping a stolen device from accessing that network by blacklisting any phone from using an IMEI number belonging to the stolen device. For example, if such a phone is stolen, the owner can call their network provider and instruct the provider to blacklist the phone using its IMEI number. This renders the phone useless on that network and sometimes other networks too, whether or not the phone's SIM (Subscriber Identity Module) card is changed. The network provider may associate with other network providers to share invalid IMEI numbers to enhance the blacklisting strategy.
IMEI numbers are regulated by GSMA (GSM Association), which assigns Type Allocation Codes (TAC) issued by GSMA. TAC are used in combination with manufacturer's codes to create an IMEI number, which is registered with GSMA to identify a maker of a phone, the type of phone, and a serial number of the phone. An IMEI number may also include a one-digit check digit or a two-digit software version number.
The IMEI number blacklisting strategy has its shortcomings. Although there is not a good reason for someone to change a phone's IMEI number—many jurisdictions even make the practice of doing so illegal—thieves will attempt to take non-blacklisted IMEI numbers and apply them to stolen devices in a practice known as “cloning.” IMEI numbers are not supposed to be easy to change, making blacklisting effective. However, this is not always the case. A phone's IMEI number may be easy to change with specialized tools, such as tools to re-flash memory that stores an IMEI number. Also, an IMEI number is an unauthenticated mobile identifier. Spoofed IMEI numbers can thwart all efforts to track devices, or target devices for lawful intercept. Also, some network providers do not block tainted IMEI numbers, making blacklisting a less than perfect solution. Furthermore, a valid user may have a device blocked if he legitimately purchases a used phone and inserts a SIM card. A network provider may block the device since the IMEI number and the SIM card do not match.